The invention is in the field of transistor logic output circuits for transistor logic devices. In particular, the invention is applicable to the field of TTL circuits employing a "totem-pole" arrangement consisting of a pull-down, current-sink transistor and a pull-up, current-source transistor. The invention increases the speed of operation of the pull-down transistor and precisely phases its operation with that of the pull-up transistor in order to reduce the total AC power consumed by the circuit.
Conventionally, a digital transistor logic output circuit provides, through an output terminal, a voltage signal whose state is determined by the state of a signal input to the circuit. In the typical totem-pole arrangement, the output voltage is developed at the collector of a current-sink transistor. As is known, the transition speeds--the rise and fall times--of the output voltage are determined by the speed with which the current-sink transistor is switched between saturation and nonconduction.
The speed of operation of the current-sink transistor is, in turn, largely affected by its base-collector capacitance. Thus, in turning on the transistor, it is preferable to supply a relatively large current to its base in order to quickly charge the capacitance and place the transistor in operation. Similarly, quickly discharging the capacitance when the base current is removed from the transistor will shorten its turn-off time.
In most prior art logic output circuits, the totem-pole arrangement is driven by a phase-splitter section which responds to the input signal. The current-sink transistor is switched on by a current signal provided to its base by the phase-splitter. The transistor is switched off by removal of the current. In the usual arrangements for speeding up the output transition times of a logic circuit, a discharge element is placed in the base circuit of the current-sink transistor. The discharge element is operated to provide a discharge path for the base-collector capacitance of the sink transistor when the signal current is removed.
It is vital to the efficient and speedy operation of the circuit that the operation of the discharge element be synchronized with that of the phase-splitter. Synchronization prevents the discharge element from providing a parallel path for the diversion of signal current when the sink transistor is being turned on. On the other hand, the diversion element must be active precisely when the input signal changes in a direction requiring that the sink transistor be turned off, in order to prevent the sink transistor from conducting while its base-collector capacitance discharges. Such conduction lengthens the output signal transition and adds to the current load passed by the pull-up element of the totem-pole circuit.
Therefore, there is an evident need in logic output devices employing a totem-pole output circuit to provide for the rapid discharge of the base-collector capacitance of the totem-pole sink transistor and to ensure that this discharge is synchronized to the operation of the pull-up transistor.